It helps to read the entire thing...at least if you want to actually understand what's happening.

This isn't about sending someone to jail over what they may or may not do to children that may or may not exist so much as it's about sending them to jail for importing obscene materials across state lines.

Obscenity of any kind outside of the privacy of the home is considered fair game for legislation by the courts and it is illegal in most if not all states. If you read the previously posted court decision the statutes the defendant is being accused under refer specifically to obscenity. Read (18 U.S.C. § 1466A for the full law. Sections (a)(1) and (b)(1) explicitly outlaw "production, distribution, receipt, or possession with intent to distribute," of material which depicts (or appears to depict) a minor AND is obscene. The judge actually struck down as unconstitutional sections 1466A(a)(2) and (b)(2) because they lacked an obscenity requirement and only required the material "lack serious literary, artistic, political, or scientific value;"(which happens to be 1/3rd of the standard obscenity requirement (see: Miller Test) but whatever.

If you read the whole decision what the judge basically says that "a jury has to decide if the material in question is obscene, the Court can not presume the jury's findings and dismiss the charges". If the manga is deemed obscene by a jury of 12 people from Iowa then that guy broke the law, if it's not deemed obscene then he didn't. The place the 'child' aspect comes in is in the sentencing. If he's convicted under 1466A the punishment is up to 10 years in prison and/or a fine, with the caveat that if he's been previously convicted of a sexually themed crime it becomes an automatic minimum 10 years in prison and up to 10 more AND a fine. In contrast, if he were to be tried and convicted under 1466 (the regular obscenity prohibition) the punishment would be up to 5 years in prison and/or a fine.

Constitutionally, obscenity is not a protected right so the states and federal government can make laws restricting it provided they stay in the bounds of their authority (the Fed can only legislate interstate trafficking of obscene materials for example).